Tree and Vine

A tree is 2 zhang tall and has a circumference of 3 chi.  There is a vine that grows from the bottom of the tree, winds seven equally spaced times around the tree and reaches the top.  What is the length of the vine? [Note: 1 zhang = 10 chi]

Jiuzhang Suanshu (The Nine Chapters on the Mathematical Art), c. 100 BCE

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